1. Field of the Invention
The present invention relates to a very high frequency tuner, and more specifically relates to an improvement in a tuned coupling circuit between a high frequency amplifier and a frequency converter in a very high frequency tuner, particularly for use in television receivers.
2. Description of the Prior Art
A typical prior art very high frequency tuner comprises a high frequency amplifier and a frequency converter, with a tuned coupling means interposed therebetween. FIG. 1 is a block diagram of a typical very high frequency tuner, in which the present invention can be advantageously employed. The tuner is shown connected to an antenna ANT for receiving a broadcasting wave and comprises a high frequency amplifying means HFA for selectively amplifying the selected high frequency signal, a local oscillator means 5 for providing an oscillation frequency signal the frequency of which is different by a given frequency difference from the selected high frequency signal, and a mixing means 4 responsive to the received high frequency signal and the local oscillation frequency signal for providing an intermediate frequency signal. The high frequency amplifying means HFA comprises a high frequency tuning circuit 1, a high frequency amplifying transistor circuit 2 and an interstage tuned coupling circuit 3. A detailed structure and operation of the high frequency tuned circuit 1, the high frequency amplifying transistor circuit 2, the mixer 4 and the oscillator 5 are well known to those skilled in the art. Hence, it is not believed necessary to describe the same in more detail for the purpose of describing the present invention. The mixer means 4 and the local oscillating means 5 are generally referred to as a frequency converter.
The present invention is directed to an improvement in the interstage tuned coupling circuit 3. As well known, one type of such an interstage tuned coupling circuit comprises a double tuned coupling circuit, i.e. a pair of inductively coupled tuned circuits and the other type of such a tuned coupling circuit comprises a single tuned coupling circuit. In comparison of these, a single tuned circuit is more advantageous from the standpoint of economy and fabrication of the same.
FIG. 2 shows a schematic diagram of a typical prior art single tuned coupling circuit employed as an interstage tuned coupling circuit between a high frequency amplifier and a frequency converter. Referring to FIG. 2, the output of the high frequency amplifier 2 implemented by a transistor TR1 and the input of the mixer 4 implemented by a transistor TR2 are coupled by means of a series connection of capacitors C1 and C4. The junction of the capacitors C1 and C4 is connected through a series connection of a capacitor C2 and a voltage controlled variable capacitance diode D1 to the ground, the polarity of the diode D1 being selected such that the cathode is connected to the capacitor C2 and the anode is connected to the ground. The junction of the capacitor C2 and the diode D1 is connected to receive a tuning control voltage through a resistor R1. The junction of the capacitors C1 and C4 is also connected through a series connection of a first and second inductance coils L1 and L2 to the ground. The junction of the inductance coils L1 and L2 is connected through a switching diode D2 and further through a resistor R2 to a source of a band selection switching control signal. The junction of the diode D2 and the resistor R2 is connected through a capacitor C3 to the ground for the purpose of decoupling.
In operation, when a low frequency band of the television channels is to be selected, a frequency band selection control signal of a higher voltage is applied through the resistor R2 to the diode D2, so that the switching diode D2 is reverse biassed and hence is turned off, with the result that a parallel tuning circuit is constituted by the capacitance across the voltage controlled variable capacitance diode D1 and the inductance coils L1 and L2, as series connected, and when a high frequency band of the television channels is to be selected a frequency band selection control signal of a lower voltage is applied through the resistor R2 to the diode D2, so that the switching diode D2 is forward biassed and is turned on, with the result that the inductance coil L2 is short circuitted and a parallel tuning circuit is constituted by a capacitance across the voltage controlled variable capacitance diode D1 and the inductance coil L1.
The FIG. 2 very high frequency tuner, however, suffered from a disadvantage that a capability of eliminating an image interference is poor on the occasion of high band reception. An image inteference may be defined as an adverse influence caused by a pseudo intermediate frequency signal resulting from a high frequency signal of a channel the frequency of which is higher than that of a channel being received. By way of an example, assuming that a broadcasting channel of the central frequency f1 is to be received, then the local oscillator 5 is adapted such that the local oscillation frequency f0 satisfies the relation f0-f1=fi with respect to the central frequency f1 of the broadcasting channel where fi is the intermediate frequency and is 56.5 MHz and as a result an intermediate frequency signal based on the desired broadcasting channel is obtained and applied to the succeeding intermediate frequency circuit; however, if and when a high frequency signal of a broadcasting channel the central frequency f2 of which satisfies the relation f2-f0=56.5 MHz is transmitted from the high frequency amplifying transistor TR1 to the frequency converting transistor TR2, an intermediate frequency is produced based on the above described broadcasting channel of the higher frequency f2, which causes a noise on the screen of the television receiver. Accordingly, a consideration is required to prevent such higher frequency channel signal from being transferred to the frequency converting transistor TR2. It is also required to prevent an interference by virtue of any lower frequency signal such as an intermediate frequency signal as well as an interference by virtue of the signal of a frequency higher than a frequency being received.